You can move a set of time entries to another project by using the Batch Update feature on the Project ID of the time entries. Check CORE Help Center for details.
Note: If you are having project assignments on the target project and have assigned only some activities to it, CORE only moves those time entries whose activities are assigned to that project. You need to make sure that you assign all the activities of the time entries that you are moving to that project before doing batch update. Check CORE Help Center for details on assigning activities to projects.